To produce perforations in the rack its characteristics and use must be taken into consideration. A flexible, perforated rack consists of a tape having a cross-sectional curvature and having perforations along its length. It is thin, rigidly straight, resilient and usually made of metal. However, it may be made of plastic. Although thin is possesses considerable strength and rigidity being cross-sectionally curved. When used it is usually bent toward its convex surface about a coupling aligned with sprockets worn and mated with the rack perforations.
Since the rack will flatten laterally where it is bent radially, it may be visualized that its perforations must be produced while it is bent from its natural cross-sectionally curved position and corresponding to its being bent about the coupling. For example, if perforations are produced in a rack while it is straight and cross-sectionally curved, the shape of the perforations will change both longitudinally and transversely when the rack is bent radially and flattened laterally. Since the perforations are eventually worn with sprockets while the rack is bent thus flattened laterally, attempts to perforate the rack while in its straight and cross sectionally curved position, with possibly attempting to predetermine the perforation's shapes and spacings when bent, would be impractical in view of fabricating tolerances.